RE: The beans have been spilled!

Your friend is right. With most, if not all, DACs the differences from moment to moment are likely to be much greater than the difference between A and B. However, over a period of time these differences average out and it would certainly be possible for analog measurement apparatus to detect the differences by using sophisticated averaging techniques. What's rather more amazing is that some audiophiles can do this by ear.

If you do null tests in the analog domain you will get different results each time because the digitization of the analog output happens with a separate clock. The relative phase of the the two clocks constantly changes, invalidating the null tests. It may be possible to slave the two clocks, but jitter noise will still be different.

With ladder DACs the waveform output by a DAC will not vary from play to play, except for noise, voltage and temperature changes. With a delta-sigma DAC there is a new source of randomness, the state of the delta-sigma modulator, which is a pseudo-random number generator. If the modulator is given the same set of input bits on two separate occasions it will not produce the same set of output bits to convert to analog. That's because the modulator is a digital feedback circuit which retains a history of it's previous activity. This is easily shown to be the case if one experiments with these modulators with software. One gets a completely different pattern of output noise when feeding them two constant input signals that differ by one in the least significant bit. The ability to hear these differences (especially in the DC case) must be taken as a fault in the DAC, not a "resolving" feature.

Tony Lauck

"Diversity is the law of nature; no two entities in this universe are uniform." - P.R. Sarkar